The present invention relates to semiconductor lasers and light emitting diodes, and more particularly, to an improved p-contact with such devices.
The development of short wavelength light emitting devices is of great interest in the semiconductor arts. Such short wavelength devices hold the promise of providing increased storage density for optical disks as well as full-color displays and white light sources when used in conjunction with devices that emit light at longer wavelengths. For example, blue lasers are expected to increase the storage density of optical disks by a factor of three.
One promising class of short wavelength light emitting devices is based on group III-V semiconductors, particularly group III nitride semiconductors. As used herein, the class of group III nitride semiconductors includes GaN, AlN, InN, BN, AlInN, GaInN, AlGaN, BAlN, BInN, BGaN, and BAlGaInN. To simplify the following discussion, xe2x80x9cGaN semiconductorsxe2x80x9d includes GaN, and group III nitride semiconductors whose primary component is the GaN as in GaInN, AlGaN, BGaN, and BAlGaInN.
Light emitting diodes (LEDs) and semiconductor laser diodes are fabricated on epitaxially grown layers of GaN and related alloys of semiconductor materials including an active layer that generates light by recombining holes and electrons. The active layer is sandwiched between p-type and n-type contacts to form a p-n or n-p diode structure. A p-electrode and an n-electrode are used to connect the p-contact and n-contact, respectively, to the power source used to drive the device. The overall efficiency of the device may be defined to be the light emitted to the outside generated per watt of drive power. To maximize the light efficiency, both the light generated per watt of drive power in the active layer and the amount of light exiting from the device in a useful direction must be considered.
It should be noted that the resistance of the p-type nitride semiconductor layer is much more than the resistance of the n-type nitride semiconductor layer. The resistivity of the p-contact layer is typically 100 to 1000 times that of the n-contact nitride semiconductor. When the p-electrode is formed on the p-type nitride semiconductor layer, a Schottky junction or ohmic junction is formed. In either case, there is a voltage drop across the junction, and hence, power is wasted at the junction. In GaN blue lasers, this voltage drop can be 10-20V. The power dissipated at the p-contact is sufficient to limit the continuous power that can be generated by the device. In addition, the power dissipated at the p-contact does not generate any light; hence, the high resistivity of the p-contact layer reduces the overall efficiency of the device.
In GaN based LEDs, the p-contact is also the layer through which light is extracted from the device. The high resistivity of the p-contact material requires that the p-electrode cover substantially all of the p-contact layer since lateral current spreading is minimal. Hence, the light is forced to exit through the p-electrode. Even when very thin electrode layers are utilized, a substantial fraction of the light generated in the LED is absorbed by the p-electrode. Accordingly, the efficiency of the LED is substantially reduced.
Broadly, it is the object of the present invention to provide improved LEDs and semiconductor lasers based on group III-V semiconductors.
It is a further object of the present invention to provide light emitting devices with increased light output efficiency.
It is yet another object of the present invention to provide a p-contact structure that reduces the problems associated with the prior art structures discussed above.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a light-generating device such as a laser or LED. The light generating device includes a first n-electrode layer in contact with a first n-doped contact layer that includes an n-doped semiconductor. Light is generated in an n-p active layer in response to the recombination of holes and electrons in the n-p active layer. The n-p active layer includes a first p-doped active layer in contact with a first n-doped active layer, the first n-doped active layer being connected electrically with the first n-doped contact layer. A p-n reverse-biased tunnel diode that includes a second p-doped layer is in contact with a second n-doped layer, the second p-doped layer is connected electrically with the first p-doped active layer. A second n-electrode layer is in contact with the second n-doped layer. The first n-doped contact layer and the second n-doped layer are preferably GaN semiconductors. The p-n reverse bias tunnel diode preferably includes an n-depletion region in the second n-doped layer and a p-depletion region in the second p-doped layer, the n-depletion region and the p-depletion region being in contact with one another. In one embodiment, the n-depletion region includes a compressively strained InGaN layer.